Process to mitigate grain texture differential growth rates in mirror-finish anodized aluminum

ABSTRACT

Anodizing processes for providing durable and defect-free anodic oxide films, well suited for anodizing highly reflective surfaces, are described. In some embodiments, the anodizing electrolyte has a sulfuric acid concentration substantially less than conventional type II anodizing. In some embodiments, the electrolyte includes a mixture of sulfuric acid and one or more organic acids. In further embodiments, sulfuric acid is a relatively minor additive to an organic acid, primarily serving to minimize discoloration. The processes enables porous, optically clear, and colorless anodic films to be grown in a manner similar to conventional Type II sulfuric acid anodizing, but at lower current densities and/or higher temperatures, without compromising film surface hardness. The thickness uniformity of the resulting anodic oxide films can be within 5% between grains of {111}, {110} and {100} surface orientations. Furthermore, the anodic oxide films have minimal incorporated sulfates, thereby avoiding certain cosmetic and structural defects.

FIELD

The described embodiments relate generally to anodized films and methods for forming the same. More particularly, the present embodiments relate to methods for producing defect-free anodized films on highly polished metal substrates.

BACKGROUND

The surfaces of many products in the commercial and consumer industries may be treated by any number of processes to alter the surface and create a desired effect, either functional, cosmetic, or both. One example of such a surface treatment is anodizing of a metal substrate. Anodizing converts a portion of the metal substrate into a metal oxide, thereby creating a metal oxide layer, which is generally harder than the underlying metal substrate and therefore acts as a protective layer. A well-known anodizing method, often referred to as Type II anodizing, has been found to provide metal oxide layers with good corrosion and wear resistance for many consumer products.

The surface of the metal substrate can be treated prior to an anodizing treatment to give the substrate a desired texture. In some cases, the substrate is lapped or polished smooth, providing a mirror shine finish to substrate. It has been found, however, that on certain aluminum alloys (and notably on 7000-series aluminum), conventional Type II anodizing of a highly polished substrate can cause grain-to-grain thickness variations which give the anodized surface an “orange-peel” like texture, and in some more severe cases, can cause tiny but visible indentations or pits to form at the metal/oxide interface, corresponding to the grain structure of the underlying metal substrate. These tiny pits are scattered along the entire surface the substrate. Although these pits are very small, they can detract from the pristine look of the mirror polished substrate.

SUMMARY

This paper describes various embodiments that relate to anodizing processes and anodic oxide coatings using the same. Although they may be applied to any aluminum alloy, they are of particular relevance to certain alloys (such as the 7000-series aluminum used by Apple Inc., based in Cupertino, Calif.) where alloying elements such as zinc, copper, manganese and magnesium result in certain defects in the anodic oxide film. The methods can be used to provide durable and defect-free anodized films of great thickness uniformity, specifically less than 5% variation in thickness between grains of any surface orientation, giving improved anodic oxide cosmetics, especially on highly polished substrate surfaces.

According to one embodiment, a method of forming an anodic film is described. The method includes anodizing a substrate in an electrolyte comprising no greater than 7% sulfuric acid by weight, using a current density of no greater than 1 A/dm², such that the resultant anodic oxide film is uniform in thickness to within 5%, irrespective of the surface orientation of grains, and has a hardness of no less than 320 HV_(0.05).

According to another embodiment, a method of forming an aluminum oxide coating is described. The method includes anodizing an aluminum or aluminum alloy substrate in an electrolyte with a sulfuric acid concentration ranging between 5 g/L and 70 g/L. The electrolyte optionally includes one or more organic acids at an organic acid concentration ranging between 10 g/L and 100 g/L.

According to a third embodiment, a method of forming an anodic film is described. The method involves anodizing a substrate in an electrolyte, which is predominantly comprised of organic acid (ranging from 20 g/L to 100 g/L), with a relatively minor addition of sulfuric acid (5 g/L to 20 g/L). This electrolyte yields a colorless anodic oxide film of great thickness uniformity (less than 5% variation from grain to grain), and hardness of no less than 320 HV_(0.05), even when operated at high temperatures (25 C or higher) and/or low current densities (1 A/dm² or less).

According to a further embodiment, a metal housing for an electronic device is described. The metal housing includes an anodic film having no greater than 4% by weight of sulfur. This low sulfur content avoids interfacial adhesion problems associated with the accumulation of elements such as zinc at the interface between the metal and the oxide during anodizing. The anodic film has a hardness value of no less than 320 HV_(0.05) as measured by Vickers hardness test.

According to another embodiment, a method of forming an anodic film is described. The method includes anodizing an aluminum alloy substrate in an electrolyte using a current density of no greater than 1 A/dm² and/or an electrolyte temperature of no less than 30 degrees C. such that the resultant anodic film has a hardness value of no less than 320 HV_(0.05).

These and other embodiments will be described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements.

FIG. 1 shows consumer products than can be manufactured using anodizing methods described herein.

FIGS. 2A and 2B show cross section views of a part undergoing a conventional Type II anodizing process.

FIGS. 3A and 3B show graphs indicating hardness of anodic oxide coatings as a function of anodizing time and anodizing current density.

FIGS. 4A and 4B show cross section views of a part undergoing an anodizing process in accordance with some described embodiments.

FIG. 5 shows a flowchart indicating an anodizing process in accordance with some described embodiments.

DETAILED DESCRIPTION

Reference will now be made in detail to representative embodiments illustrated in the accompanying drawings. It should be understood that the following descriptions are not intended to limit the embodiments to one preferred embodiment. To the contrary, it is intended to cover alternatives, modifications, and equivalents as can be included within the spirit and scope of the described embodiments as defined by the appended claims.

The following disclosure relates to anodizing processes that result in cosmetically appealing and durable anodic oxide films. The anodizing processes described herein can be used as alternatives to conventional Type II anodizing processes, which have been found to cause certain visible defects associated with the grain orientations of the underlying metal substrate. The anodizing processes described herein can be used to form protective coatings without introducing these visible defects even when performed on highly visible surface, such highly polished and reflective metal surfaces.

In some embodiments, the anodizing processes include using an electrolyte with dilute concentrations of sulfuric acid compared to Type II anodizing processes. In particular embodiments, the sulfuric acid concentration is 70 g/L or less, and in some cases ranges between 5 g/L to 20 g/L. This is compared to conventional Type II anodizing electrolytes that typically have sulfuric acid concentrations ranging between 10-20% by weight. In some embodiments, the electrolyte includes a mixture of sulfuric acid with one or more organic acids. In a particular embodiment, the total concentration of organic acid within the electrolyte ranges between 10-100 g/L. In a further embodiment, the electrolyte mixture is predominantly comprised of organic acid (20 g/L to 100 g/L) with sulfuric acid as a relatively minor additive (5 g/L to 20 g/L).

Because the electrolyte has a lower concentration of sulfuric acid, it dissolves the anodic oxide film during the anodizing process at a lower rate than a conventional sulfuric acid electrolyte, which enables porous, optically clear, and colorless films to be grown in a manner similar to conventional Type II sulfuric acid anodizing, but at lower current densities (1 A/dm² or lower) and/or higher temperatures (25-40 degrees C.), without compromising the metal oxide film surface hardness relative to conventional type II sulfuric acid (specifically, the about 320 HV_(0.05) hardness measured on films grown to 10 micrometers thickness at 20 degrees C. and 1.5 A/dm² in 200 g/L sulfuric acid). The lower sulfuric acid concentration electrolytes can also result in minimal incorporation of acid anions into the particularly when anodizing is also performed at relatively high temperatures (e.g., 30 or 35 degrees C.) and/or relatively low current densities (e.g., no greater than 1 A/dm2). Thus, the anodic oxide films can have sulfur concentrations of less than 4% by weight. This can be of particular benefit in avoiding a propensity for low interfacial adhesion of anodic oxides to 7000-series aluminum alloys (where zinc enrichment occurs at the oxide interface, combining with sulfur to weaken the interface).

The anodizing methods described herein can be applied to substrates made of any suitable anodizable material. Although particular reference is made to 7000-series aluminum alloys, and to alloys comprising zinc, copper, manganese and magnesium, the method could be applied to other aluminum alloys where similar mechanisms of differential growth rates on different grain orientations occur, or where interfacial enrichment of alloying elements weakens an anodic oxide adhesion. As described herein, the terms “anodic film,” “anodic oxide,” “anodic layer,” “anodic oxide,” “anodic oxide film,” “anodic oxide layer,” “anodic oxide coating” “metal oxide,” “metal oxide film,” “metal oxide layer,” and “metal oxide coating” can be used interchangeably.

Methods described herein are well suited for providing cosmetically appealing surface finishes to consumer products. For example, the methods described herein can be used to form durable and cosmetically appealing finishes for housing for computers, portable electronic devices and electronic device accessories, such as those manufactured by Apple Inc., based in Cupertino, Calif.

These and other embodiments are discussed below with reference to FIGS. 1-5. However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these Figures is for explanatory purposes only and should not be construed as limiting.

Methods described herein can be used to form durable and cosmetically appealing coatings for metallic surfaces of consumer devices. FIG. 1 shows consumer products than can be manufactured using methods described herein. FIG. 1 includes portable phone 102, tablet computer 104 and portable computer 106, which can each include metal surfaces. Devices 102, 104 and 106 can be subject to impact forces such as scratching, dropping, abrading, chipping and gouging forces during normal use. Certain alloys (such as 7000-series aluminum) are selected for making the enclosures of such devices, often driven by requirements such as high strength and hardness. Metal surfaces of devices 102, 104 and 106 are typically anodized in order to add a protective anodic oxide coating to these metal surfaces. However, it has been found that use of conventional Type II anodizing process can cause visual defects on these anodized surfaces, detracting from the aesthetic appeal of devices 102, 104 and 106. These visual defects can be particularly severe with higher strength or hardness alloys. If the metal surfaces are highly polished and reflective, these visual defects can be even more apparent. In addition, depending on the type of metal alloys used, conventional Type II anodizing can result in an anodic oxide coating that is prone to chipping and scratching from impact forces. The anodizing methods described herein provide anodic oxide coatings having improved cosmetic qualities and resistance to chipping and delamination compared to anodic oxide coatings formed using conventional Type II anodizing processes.

Aluminum and aluminum alloys can exhibit a highly reflective surface when lapped or polished to a smooth finish. This mirror-like finish may be protected against abrasive wear by applying a substantially transparent anodic oxide, such as that formed by Type II sulfuric acid anodizing (or simply Type II anodizing) in accordance with the Aluminum Anodizing Council's (AAC) Military Specification Mil-A-8625. Using Type II anodizing on certain alloys of aluminum, however, can create defects related to the different crystallographic orientations of the grains within the aluminum substrate.

To illustrate, FIGS. 2A and 2B show cross section views of part 200 undergoing a conventional Type II anodizing process. FIG. 2A shows part 200, which includes metal substrate 201, prior to anodizing. Metal substrate 201 can be made of any suitable anodizable material, typically an aluminum alloy. The following description is particularly relevant to high strength aluminum alloys (such as 7000-series aluminum), where alloying elements such as zinc, copper, manganese and magnesium result in anisotropic anodizing behavior. Surface 202 of metal substrate 201 can be lapped or polished to a mirror shine. Metal substrate 201 has grains 206 a, 206 b, 206 c, 206 d and 206 e along surface 202 that are defined and separated by grain boundaries 204. Grains 206 a, 206 b, 206 c, 206 d and 206 e are inherent crystallographic structures having different crystallographic orientations within metal substrate 201. Grains 206 b and 206 d have {111} crystallographic orientations while grains 206 a, 206 c and 206 e have crystallographic orientations that are different than {111}, such as {110} and {100} crystallographic orientations. Grains 206 b and 206 d having {111} crystallographic orientations are generally dispersed throughout metal substrate 201. The size and distribution of grains 206 a, 206 b, 206 c, 206 d and 206 e can vary depending on the type of metal and the temper of metal substrate 201.

FIG. 2B shows metal substrate 201 after a conventional Type II anodizing process. Anodizing processes, in general, involve converting a portion of metal substrate 201 to a corresponding metal oxide, referred to as anodic oxide coating 208. Thus, metal substrate 201 can be referred to as an underlying metal substrate 201. Interface 210 between anodic oxide coating 208 and metal substrate 201 takes on the same geometry of surface 202 prior to the anodizing process. Thus, interface 210 takes on the polished, mirror shine, highly reflective quality of surface 202. In some cases, anodic oxide coating 208 is transparent to some of the light incident surface 212 of anodic oxide coating 208 such that the highly reflective surface of interface 210 is visible through anodic oxide coating 208.

However, due to their {111} orientation in aluminum alloys having zinc, grains 206 b and 206 d undergo the conversion process faster than grains 206 a, 206 c and 206 e. Thus, anodic oxide coating 208 is thicker a locations corresponding to grains 206 b and 206 d. In some cases, it was found that grains 206 b and 206 d having {111}, or near {111} orientation, are anodized about 20% faster than grains 206 a, 206 c and 206 e having different orientations. Note that the grain orientations that experience accelerated growth are not limited to grains having {111} orientation. For example, it has been found that {111} oriented grains experienced accelerated growth in zinc-rich aluminum alloys, and {110} oriented grains experienced accelerated growth in copper-rich aluminum alloys.

The localized thicker anodic oxide coating manifests as protrusions 214 of surface 212 of anodic oxide coating 208, and indentations or pits 216 within the reflective surface of interface 210 corresponding to intrusions of anodic oxide coating 208. Pits 216 can have sizes matching the sizes of corresponding grains 206 b and 206 d. Pits 216 may be widely and asymmetrically distributed over the surface interface 210 with a bimodal or tri-modal distribution, and a distribution that widens with increasing thickness of anodic oxide coating 208. The variations in oxide thickness from grain to grain are visually perceived as an “orange peel” texture in the metal/oxide interface, detracting from the smooth, mirror-like reflective quality of the surface of interface 210. Specifically, pits 216 can appear as scattered tiny bright spots that interrupt the mirror-like appearance of part 200 as viewed from surface 212. In this way, pits 216 can be referred to as visible defects within part 200. These visible defects can become very noticeable when the average thickness of anodic oxide coating 208 exceeds about 6 micrometers. For many applications, however, the thickness of anodic oxide coating 208 should be greater than about 10 micrometers in order to provide good wear protection. Thus, these visual defects would be very apparent in these applications. It should be noted that if substrate 201 had a rougher texture, such as a blasted finish, or if anodic oxide coating 208 is dyed, pits 216 may be visually apparent as a general non-uniformity or blotchiness as viewed from surface 212, which is also undesirable.

A further, un-related problem with using conventional Type II anodizing with certain alloys is weakened interfacial adhesion as a result of interfacial accumulation of alloying elements such as zinc, which can combine with sulfur from the sulfuric acid of the Type II electrolyte. These sulfur-containing agents can weaken the bonding strength at interface 210 between substrate metal substrate 201 and anodic oxide coating 208. This is described in related U.S. patent application Ser. No. 14/474,021 filed Aug. 29, 2014, which is incorporated by reference herein in its entirety.

One approach to mitigating the problem of differential growth anodic oxide growth rates on grains of different orientations is to limit the applied current density to less than 1.0 A/dm², or to limit the applied voltage to less than 10 V. However, these conditions limit the hardness and durability of the resulting anodic oxide coating due to the excessive dissolution of the anodic oxide material by the sulfuric acid during the prolonged exposure that is required to grow an anodic oxide coating of sufficient thickness (e.g., 10 micrometers or more). That is, the resultant anodic oxide coating will not be hard enough to provide sufficient abrasion and wear resistance for many consumer products, especially at edges and corners of the consumer product. To illustrate, FIGS. 3A and 3B show graphs indicating hardness of an anodized 7003 aluminum substrate as a function of anodizing time and current density using a conventional Type II anodizing process.

The graph of FIG. 3A shows hardness data for anodic oxide coatings grown to 9 micrometers in thickness using Type II anodizing. This data shows that the hardness of an anodic oxide coating decreases with increased anodizing processing time. This is due to the dissolution of the anodic oxide material during the anodizing process. The graph of FIG. 3B shows hardness data for anodic coatings grown to 10 micrometers thickness using Type II anodizing, with anodizing process times indicated. This data shows that, for a given film thickness, lower current density necessitates increased anodizing process time and results in reduced surface hardness. For a number of applications, a current density around 1.5 A/dm² or higher is necessary in order to provide a sufficiently durable anodic oxide coating. In order to reduce the occurrence of the above-described pit defects, the current density would have to be reduced to about 0.5 A/dm², which would result in a soft anodic oxide film that is not hard enough for many consumer product applications.

The methods described herein address the above-described issues associated with using conventional Type II anodizing processes. The methods involve reducing the dissolving power of the sulfuric acid electrolyte during the anodizing process by reducing the concentration of sulfuric acid within the anodizing electrolyte. FIGS. 4A and 4B show cross section views of part 400 undergoing an anodizing process in accordance with some embodiments. FIG. 4A shows part 400, which includes metal substrate 401, after an optional surface finishing process. The optional surface finishing process can include lapping, polishing and/or buffing of surface 402. In some cases, surface 402 is polished to a mirror-like shine. That is, surface 402 can be highly reflective to incident light. In other embodiments, surface 402 is treated to have a rough texture, such as by a blasting operation and/or etching operation. Metal substrate 401 can be made of any suitable anodizable material, such aluminum or aluminum alloy. In some cases, substrate is made of an aluminum alloy with zinc, magnesium and/or copper alloying elements. Metal substrate 401 has grains 406 a, 406 b, 406 c, 406 d and 406 e defined and separated by grain boundaries 404. Grains 406 b and 406 d have {111} crystallographic orientations that undergo accelerated anodizing using Type II anodizing conditions, described above. Grains 406 a, 406 c and 406 e have different crystallographic orientations than grains 406 b and 406 d and do not undergo accelerated anodizing using Type II anodizing conditions.

FIG. 4B shows part 400 after an anodizing process, where a portion of metal substrate 401 is converted to a corresponding metal oxide, referred to as anodic oxide coating 408. If metal substrate 401 is aluminum or aluminum alloy, anodic oxide coating will include aluminum oxide. Remainder portion of metal substrate 401 is positioned below anodic oxide coating 408, and thus can be referred to as an underlying metal substrate 401. Interface 410 between anodic oxide coating 408 and metal substrate 401 takes on the same geometry of surface 402 prior to the anodizing process. Thus, interface 410 takes on the polished, mirror shine, highly reflective quality of surface 402. In some embodiments, anodic oxide coating 408 is transparent to at least some of the light incident surface 412 of anodic oxide coating 408 such that the highly reflective surface of interface 410 is visible through anodic oxide coating 408.

Anodic oxide coating 408 is formed using an anodizing process with an electrolyte having a lower concentration of sulfuric acid compared to electrolytes used in Type II anodizing. The lower concentration of sulfuric acid reduces the dissolving power of the sulfuric acid within the electrolyte and thereby produces a harder anodic oxide coating 408. In addition, since the sulfuric acid concentration is less than Type II anodizing, accelerated anodizing due to different grain orientation is reduced or eliminated. Thus, the thickness of anodic oxide coating 408 grown at {111} oriented grains 406 b and 406 d will be substantially the same as the thickness of anodic oxide coating 408 at grains 406 a, 406 c and 406 e. In this way, the above-described pits from using Type II anodizing is dramatically reduced or eliminated and the thickness of anodic oxide coating 408 is more uniform than that of an anodic oxide coating formed using Type II anodizing. That is, substrate 401 is substantially free of indentations and the highly reflective surface at interface 410 remains uninterrupted and pristine in appearance.

The concentration of sulfuric acid can vary depending on a desired hardness and reduction of pit defects. In some embodiments where substrate 201 is made of an aluminum alloy, the sulfuric acid concentration was reduced to less than about 70 g/L, or less than about 7% by weight. In some embodiments the sulfuric acid concentration ranged between about 50-60 g/L. In other embodiments, a sulfuric acid concentration as low as about 5 g/L is found to be sufficient. These are well below any recited literature for Type II anodizing electrolytes. For example, conventional Type II anodizing typically includes using an electrolyte having a sulfuric acid concentration of ranging between about 180-210 g/L, or about 10-20% by weight.

The rate of dissolution of the anodic oxide coating 408 during anodizing is significantly lower in the lower sulfuric acid electrolytes than in conventional Type II electrolytes. This reduced rate of anodic oxide dissolution results in lower surface porosity and greater surface hardness of anodic oxide coating 408 compared to anodic oxides grown to equivalent thicknesses in Type II electrolytes, even when the current density or growth rates for the latter is four or five times higher. In this way, the dilute sulfuric acid concentration electrolyte enables an anodizing process with results similar to more conventional Type II anodizing. That is, the resultant anodic oxide coating 408 is a reasonably hard (i.e., ≧320 HV_(0.05)), clear, porous oxide film, which is also well suited to dyeing and sealing processes. As known in the art, HV_(0.05) refers to a Vickers hardness testing scale, specifically at a load of 50 g. This may be measured on a polished surface, or directly on an anodized surface when that same has been formed on a polished substrate. It is recognized that at thicknesses of 10 micrometers or less, contributions from the substrate hardness will have an influence on the measured surface hardness, and the measured value may not reflect the true, absolute hardness of corresponding bulk material. However, throughout this paper, quoted hardnesses are measured in the same way, allowing meaningful comparisons of relative hardness values.

In some embodiments, the electrolyte includes other acids, such as one or more organic acids. It has been found in some cases that adding an organic acid to the electrolyte can increase the hardness of the final anodic oxide coating 408. However, organic acids can also affect the appearance of the anodic oxide coating 408, such as give anodic oxide coating 408 a yellow, gold, bronze or brown hue depending on the type and amount of organic acid. Therefore, the use or organic acid and the type of organic acid will depend on various factors such as a desired final hardness and color of anodic oxide coating 408. In some cases, suitable organic acids include one or more of oxalic acid, citric acid, malic acid, malonic acid, glycolic acid, acetic acid and tartaric acid. Operating voltages for a 0.5-2 A/dm² current density anodizing in a mixed (dilute sulfuric acid and organic acid) electrolyte can be similar to those of conventional Type II anodizing (e.g., 5-30 V, sometimes preferably 10-25 V), rather than the higher voltages typically required for anodizing in more conventional organic acid electrolytes in the absence of the sulfuric acid. In particular embodiments, oxalic acid added at a concentration of between 10-100 g/L is found to provide good hardness without too much discoloration. In some embodiments, an oxalic acid concentration of between 10-30 g/L is preferable. In some embodiments, other organic acids or mixtures of organic acids can be added to a dilute sulfuric acid electrolyte at similar concentrations. In a particular embodiment, sulfuric acid is added as a relatively minor additive (e.g., 5 g/L to 20 g/L) to an organic acid (at 20 g/L to 100 g/L), so as to reduce discoloration to negligible degree (i.e., each of a* and b*<1, as measured in accordance with CIE 1976 L*a*b* color space techniques), enabling the use of an organic acid (and the corresponding benefits of high hardness at low current density or high anodizing temperature, and minimal sulfate anion incorporation), without the usual problem of discoloration associated with anodizing in an organic acid.

The lower anodic oxide dissolution rate using dilute sulfuric acid or mixed electrolyte makes it possible to extend the range of anodizing process parameters to include lower current densities (e.g., 1 A/dm² or lower), and/or higher electrolyte temperatures (e.g., 25° C. to 40° C.) whilst maintaining anodic oxide coating 408 surface harnesses equal to or better than those achieved with Type II anodizing under more conventional conditions, such as the 320 HV_(0.05) achieved with 10 micrometer oxide growth at 1.5 A/dm² and 20° C. This expansion of the processing parameter window to lower current densities, or to higher temperatures, without sacrificing surface hardness relative to conventional Type II anodizing, enables tuning of the anodizing process to give anodic oxide coating 408 a high degree of clarity (transparency) and great thickness uniformity across surfaces comprising grains of varying crystallographic orientations (specifically less than 5% thickness variation between the film formed on grains of {111}, {110} and {100} orientation). This enables the mirror-like finish of substrate 401 to be protected against abrasion or wear with minimal loss of reflection specularity, gloss, or distinctness of image. The gloss, measured at 20 degrees, on a given lapped surface, is in excess of 1300 gloss units, when anodizing is performed in the preferred embodiments herein, whereas the more typical electrolyte used in a type II process (e.g., 200 g/l sulfuric acid) yields a maximum of about 1100 gloss units on an equivalent lapped surface at 1 A/dm² and 20 degrees C. To match the 1300 gloss units, the conventional sulfuric acid's temperature would have to be raised to 25 degrees C., and the current density lowered to 0.5 A/dm², with a resulting surface hardness reduced to about 250 HV_(0.05).

The expansion of anodizing process parameters to lower current densities or to higher temperatures, without sacrificing hardness, is also of benefit in minimizing anion incorporation into anodic oxide coating 408. The reduced concentration of sulfuric acid also helps in this regard. A purer anodic oxide coating 408 results, with less incorporation of organic acid anions (such as oxalates in cases where the acid includes oxalic acid) than would be encountered when anodizing in the pure organic acid, and also with less incorporation of sulfate ions than that encountered when anodizing in more conventional and more concentrated sulfuric acid electrolytes. This is itself of benefit in terms of increasing the hardness anodic oxide coating 408 since the incorporation of sulfate anions can compromise the hardness of the resulting oxide film. In addition, this provides benefits in terms of the clarity and optical transparency of anodic oxide coating 408. In some cases, it may also be of benefit chemically by minimizing interactions of undesirable compounds (e.g., oxalates or sulfates) with other chemicals during subsequent processing operations (e.g., dyeing and sealing of the oxide film), or during use of part 400. For example, corrosion can be minimized, and the leaching of compounds such as oxalates to skin contacts during use of part 400 may be minimized.

By minimizing organic acid anion incorporation, this approach of using a mixed acid electrolyte enables clearer oxides to be produced than would result from the organic acid components alone. For example, anodizing in an electrolyte having a concentration of about 30 g/L oxalic acid can result in yellow discoloration of the anodic oxide coating 408, whereas the addition of 5 g/L to 20 g/L of sulfuric acid results in a clear, colorless anodic oxide coating 408 In particular embodiments, the colorlessness of anodic oxide coating 408 is measured as having an a* of <1 and a b* of <1, as measured in accordance with CIE 1976 L*a*b* color space. This is desirable in many cosmetic anodizing operations, where a clear anodic oxide coating 408 is preferred, either for use in its own right, or as a neutral base color for subsequent coloration using dyes.

Similarly, by minimizing the incorporation of inorganic acid anions, relative to more conventional sulfuric acid anodizing processes, this approach of using a mixed acid electrolyte enables anodic oxide coating 408 to be formed on alloys such as AA7003, without delamination risk that would otherwise result from interactions of sulfates with zinc enriched at interface 410. Details regarding the relationship between sulfur/sulfates and delamination are provided in the U.S. patent application Ser. No. 14/474,021 referenced above. As a result, the level of sulfur in the resulting anodic oxide coating 408 can be less than 4% by weight, in some cases less than 3% by weight. This is compared to anodic oxide coatings formed using conventional Type II anodizing that generally have sulfur concentrations of greater than 10% by weight, more typically about 13% by weight.

For many cosmetic applications, anodic oxide coating 408 grown in the dilute sulfuric acid electrolyte can exhibit uniform pore structure similar to anodic oxide coating using Type II anodizing. Thus, anodic oxide coating 408 is suitable for permeation by dyes or other colorants, making it possible to achieve a wide spectrum of colors through post-anodizing operations. Moreover, due to the reduced dissolving power of the electrolyte for the growing anodic oxide material of anodic oxide coating 408 during anodizing, the outermost surface of the anodic oxide coating 408 may present an even more uniform pore structure than that of a film grown in Type II anodizing electrolyte at a given temperature. This ensures uniformity of color of anodic oxide coating 408, even when a very light dye is applied.

A further possible benefit of the anodizing processes described herein is that they may reduce in-process corrosion of certain corrosion-sensitive alloys. In particular, the increased pH, reduced sulfate concentration, and possible inhibitive action of certain organic acids such as tartaric acid may all contribute to this benefit, as may the reduced potential or local over-potentials associated with anodizing at a lower applied voltage or current density.

Table 1 below summarizes a comparison of a sample (1) anodized using a conventional Type II sulfuric acid anodizing process (as exemplified by a very typical 1.5 A/dm² process at 20 degrees C. in 200 g/L sulfuric acid) to samples (2), (3) and (4) anodized using improved anodizing processes according to some embodiments described herein. Sample (2) was anodized using dilute sulfuric acid electrolyte with no organic acid. Sample (3) was anodized using a mixed electrolyte having a concentration of 60 g/L of sulfuric acid and 30 g/L of oxalic acid. Sample (4) was anodized using a mixed electrolyte having a concentration of 10 g/L of sulfuric acid and 30 g/L of oxalic acid.

Table 1 shows that the anodizing processes used for samples (2), (3) and (4) can be performed using lower current density and/or increased temperature compared to sample (1) using conventional Type II anodizing, and still result in an anodic oxide coating having a surface hardness of 320 HV_(0.05) or greater. In particular, a current density no greater than 1 A/dm², in some embodiments 0.75 A/dm², and electrolyte temperatures of up to 35 degree C., in some embodiments up to 40 degrees C., can be used. Reduced current density, and to some degree the increased temperature, reduces the grain-to-grain thickness variation. This is exemplified by the column in Table 1 indicating Thickness Variation of a resultant anodic oxide coating formed on a 7005 aluminum alloy (AA7003), with the thickness variation measured across the substrate surface having {111} grain and a {100} grain orientations. As indicated, sample (2) have a thickness variation of 4% and samples (3) and (4) each have a thickness variation of 2%, compared to 20% thickness variation of Type II sample (1). This very small thickness variation improves the cosmetics of the anodized surfaces, as described above.

Table 1 also shows that the sulfur content within the anodic oxide coatings formed using dilute sulfuric acid electrolyte (2) and mixed acid electrolytes (3) and (4) is generally much less than that of anodic oxide coating formed using Type II anodizing (1), which correlates to improved adhesion and reduced risk of delamination for certain metal alloys, notably those such as 7000 series aluminum alloys where zinc accumulates at the interface. In some embodiments, the anodizing process is tuned to result in an anodic oxide coating having a sulfur concentration of less than 4% by weight, in some cases 3% by weight or less.

TABLE 1 Current Surface Thickness S Conc. Density Temp. Hardness variation Incorporation (g/L) (A/dm²) (° C.) (HV_(0.05)) (%) (Wt %) 1 Type II H₂SO₄ 200 1.5 20 320 20 10 2 Dilute H₂SO₄  60 ≦1 20 320 4 <4 3 Dilute H₂SO₄ + 60/30 ≦1 30 350 2 <4 oxalic acid 4 Dilute H₂SO₄ + 10/30 ≦1 35 320 2 ≦3 oxalic acid

FIG. 5 shows flowchart 500 indicating an anodizing process in accordance with some described embodiments. At 502, an optional pre-anodizing surface treatment process is performed on a substrate. The surface treatment process can include one or more of lapping, polishing, buffing, blasting, chemical etching and laser etching processes. In some embodiments, the surface of the substrate is lapped and or polished to a mirror shine such that the surface of the substrate is highly reflective of incident light. In some embodiments, the substrate is made of an aluminum alloy, such as aluminum alloys containing zinc and/or copper alloying agents. In some embodiments, the substrate is made of a 6000 series or 7000 series aluminum alloy.

At 504, the substrate is anodized in a dilute sulfuric acid electrolyte. The concentration of sulfuric acid of the electrolyte is sufficiently low to prevent formation of visually apparent defects caused by accelerated anodic oxide growth at certain grain orientations of the substrate using Type II anodizing electrolytes. That is, the anodized substrate is free of the scattered tiny pits observed on substrates anodized using Type II anodizing processes. Thus, if the substrate has a mirror shine prior to anodizing, the anodized substrate will retain the uninterrupted mirror shine. In some embodiments, the sulfuric acid concentration of the electrolyte is no greater than 7% sulfuric acid by weight. In some embodiments, the dilute sulfuric acid electrolyte includes organic acid to enhance the hardness of the resulting anodic oxide coating. In some embodiments, the resultant anodic oxide coating has a hardness of no less than 320 HV_(0.05), in some cases no less than 400 HV_(0.05). In some embodiments, the anodic oxide coating has an average concentration of sulfur of no greater than 4% by weight. For certain alloys, particularly those where zinc becomes enriched at the metal/oxide interface, reduction of the sulfur concentration within the oxide to the level is a necessary to avoid a weakened interface between the oxide and the metal, and to ensure that the anodic coating is resistant to delamination and chipping.

At 506, a post-anodizing process is optionally performed on the anodic oxide coating. The post-anodizing process can include a coloring process whereby the anodic oxide coating is dyed to a predetermined color. In some embodiments, the pore structure (e.g., pore size and pore uniformity) can be similar to the pore structure of an anodic oxide coating formed using a Type II anodizing process. Thus, the coloring process can be similar to one used in a Type II anodic oxide coating. Any suitable coloring process can be used, including organic dye infusion and/or electrolytic coloring. In some embodiments, the anodic oxide coating is sealed using a suitable pore sealing process.

The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the described embodiments. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the described embodiments. Thus, the foregoing descriptions of the specific embodiments described herein are presented for purposes of illustration and description. They are not meant to be exhaustive or to limit the embodiments to the precise forms disclosed. It will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings. 

What is claimed is:
 1. A method of forming an anodic film, the method comprising: anodizing an aluminum alloy substrate in an electrolyte using a current density of no greater than 1 A/dm² and/or an electrolyte temperature of no less than 30 degrees C. such that the resultant anodic film has a hardness value of no less than 320 HV_(0.05) and a thickness variation of less than 5% between the anodic film on grains having {111}, {110} and {100} crystallographic orientations.
 2. The method of claim 1, wherein the electrolyte has a sulfuric acid concentration no greater than 7% by weight.
 3. The method of claim 1, wherein the electrolyte comprises sulfuric acid and an organic acid.
 4. The method of claim 3, wherein after anodizing, the anodic film has an average concentration of sulfur of no greater than 4% by weight.
 5. The method of claim 3, wherein the organic acid comprises at least one of oxalic acid, glycolic acid, tartaric acid, malic acid, citric acid, and malonic acid.
 6. The method of claim 3, wherein the electrolyte has a higher concentration of organic acid compared to sulfuric acid.
 7. The method of claim 6, wherein the organic acid is oxalic acid, wherein the electrolyte has an oxalic acid concentration of at least 20 g/L and a sulfuric acid concentration between 5 g/L and 20 g/L, and wherein the resultant anodic film is grown to a thickness of at least 10 micrometers and is colorless with an a* of less than 1 and a b* of less than 1 as measured in accordance with CIE 1976 L*a*b* color space.
 8. The method of claim 1, wherein the aluminum alloy substrate comprises zinc, copper and/or magnesium.
 9. The method of claim 1, wherein the current density is no greater than 0.75 A/dm².
 10. The method of claim 1, wherein the resultant anodic film is grown to a thickness of at least 10 micrometers and is colorless with an a* of less than 1 and a b* of less than 1 as measured in accordance with CIE 1976 L*a*b* color space.
 11. A method of forming an aluminum oxide coating, the method comprising: anodizing an aluminum or aluminum alloy substrate in an electrolyte with a sulfuric acid concentration ranging between 5 g/L and 70 g/L, wherein the electrolyte optionally includes one or more organic acids at an organic acid concentration ranging between 10 g/L and 100 g/L.
 12. The method of claim 11, wherein the electrolyte includes an organic acid, and wherein the electrolyte has a higher sulfuric acid concentration than organic acid concentration.
 13. The method of claim 11, wherein sulfuric acid concentration ranges between 5 g/L to 20 g/L and the organic acid concentration is at least 20 g/L, resulting in a colorless aluminum oxide coating with an a* of less than 1 and a b* of less than 1 as measured in accordance with CIE 1976 L*a*b* color space, and wherein the aluminum oxide coating is grown to a thickness of 10 micrometers or greater.
 14. The method of claim 11, wherein anodizing the aluminum or aluminum alloy substrate comprises anodizing using a current density no greater than 1 A/dm².
 15. The method of claim 11, wherein anodizing the aluminum or aluminum alloy substrate comprises anodizing using a temperature ranging between 25° C. and 40° C.
 16. The method of claim 11, wherein after anodizing, the aluminum oxide coating has a hardness value of no less than 320 HV_(0.05).
 17. The method of claim 11, wherein after anodizing, the aluminum oxide coating has an average concentration of sulfur of no greater than 4% by weight.
 18. The method of claim 11, further comprising forming a highly reflective surface on the aluminum or aluminum alloy substrate, wherein after the anodizing the highly reflective surface is visible through the aluminum oxide coating and is uniform in thickness to within 5% on aluminum alloy substrate grains having {111}, {110} and {100} crystallographic orientations such that an interface between the aluminum or aluminum alloy substrate and the aluminum oxide coating is substantially free of indentations.
 19. A metal housing for an electronic device, the metal housing comprising: an aluminum alloy substrate comprising copper, zinc and/or magnesium; and an anodic oxide comprising no greater than 4% by weight of sulfur, wherein the anodic oxide has a hardness value of no less than 320 HV_(0.05).
 20. The metal housing of claim 19, wherein the anodic oxide has a thickness variation of less than 5% on aluminum alloy substrate grains having {111}, {110} and {100} crystallographic orientations. 